My 2 cents
(and first things first, which game?)
1...Does it help to randomly speed up or slow down my line to throw off enemy gunners?
Not really. Ships don't exactly go from 0 to 60 in a few seconds and are big targets. Speed changes are slow and the gunners will adjust pretty quickly. So don't count on speed to save you.
2. Similarly should I 'zig zag' a bit to throw them off as well...or does this just screw my own gunners up?
It does both, really, so it's your choice. I would say keeping your line steady has a more substantial effect overall. The better your gunnery, the quicker you'll put the other guy out of battle. I always keep my line on heading for as long as possible.
3...Is it better to steam at high speed...or maybe better to go slower so that more crew are available as damage control parties?
Well, this really depends on the situation. I would definitely advise going slower by a few knots in RJW where the battle is sheer slogging at closer ranges and fighting fires is extremely important. However at the same time you don't want to give your enemy a chance to cross your T and otherwise have initiative in manuevering. So choose wisely. In Jutland, the situation is generally such that, well, "speed is life" - especially for battlecruisers. Ships don't burn very well in WWI, so there's usually no reason to slow down. For the battle line, speed is initiative.
4....Where are my cruisers in all this? Behind the lines? Between them?(yikes)
Well, that depends on what you are doing. Actually "between" probably covers it - a cruiser's first role is as a scout, but once a battle is engaged, the second role comes in: their job is to beat off enemy torpedo attacks, and try to make their own. But as in any attack, surprise is the key element, so you don't want them to be haplessly floundering between two lines.
A good way to use cruisers and destroyers is to try to set them up at the head and tail of your battle line, and maybe slightly behind it. The cruisers at the head keep a watch for opportunities or threats emerging from the enemy battle line, and can leap into the thick of it when needed. The cruisers at the tail watch for stragglers and damaged ships falling out of line.
The rule of thumb with cruisers is that you never want them under steady fire by anything bigger than they are. They will go down very quickly. Cruisers are not your infantry, they're your light cavalry. If they're taking fire from the enemy battle line, the only reason they should be is that they're a) charging in to launch a torpedo attack; or b) getting out fast. However, noone says you shouldn't use your cruisers as a distraction. In fact there's nothing quite like a massed torpedo attack by destroyers and cruisers to break up the enemy line's formation, which could give your own line a powerful advantage - but often at a big cost. It would be VERY unwise to make such a charge right at the start of the battle when the enemy battle line is undamaged, has high manuevering ability and all their teeth intact. A cruiser charging into a solid battle line will end up like the Defense at Jutland. Unless there are other factors (like visibility - if you're able to get into torpedo launching range before the enemy can fire on you), I would save charges by light forces until the enemy battle line has taken a bit of a beating. You will still probably take heavy losses, but you can probably do a lot more damage. And sometimes you don't need to do damage - if a charge causes the enemy to turn or break up their battle line, it may well be worth losing a cruiser or two for in the long run. That's the nature of light forces: they run a high risk of having a short life in fleet battles. This doesn't mean they need to be your suicide squadron, but if you want them to play any role in a fleet battle, you have to accept the guarantee that there will be losses.
5.....Any other neat tactics to use?
Learn when and how to break off, turn away, or reverse direction. And leave yourself room to do it. If your enemy's force is about equal to yours as you said at the start, avoid doing what the AI always likes to do - charge in with the whole force and try to win with raw power. Keep your distance instead, try to manuever your force to take advantage of wind direction and light conditions. Pull back or get out if it's not looking to your advantage, and cover your retreat/regrouping with light forces.
Be wise about the distribution of fire. On the one hand remember that there is a heavy penalty on accuracy if multiple ships shoot at the same target, especially at long range, and conversely - if an enemy ship in the line is left untargeted, that ship will have a much easier time with gunnery. On the other hand, 1-to-1 fire distribution is not really a solution either. What I would recommend doing is at the start of the battle, distributing your fire as evenly as possible, and making sure that you do get every enemy ship under fire at first. That should put them under some stress. Then after some time, try to actually focus your fire and get more than one ship against a single target. Try to pick a ship that already looks like it took a beating, and give it more. Shift away from it as soon as it looks like it's falling out of line. Repeat with others as necessary, but don't leave any enemy ships in line untargeted for very long.
Lastly, my own rule of thumb on ranges to keep for battle lines in normal conditions: in RJW - 6-7km, in Jutland - 16-18km. The gunnery at these ranges will not be the most accurate, but this is the range at which there is still some breathing space for you to make decisions. Once you start getting closer, your risk of making wrong choices which you can't un-do increases dramatically.
***
Like I said, just my own take based on experience.
Good luck!