A lot of the answers to your questions depends on the tech you have, really, but some general advice:
Your main cmobatants will generally be missile boats, though if you have awesome beam tech, stealthed beam ships(with a separate sensor scout) do well. That said, having a few beam escorts helps when an enemy manages to close with your ships.
BTW, having specialized beam and missile ships works better in the long run than a combined ship-especially at smaller sizes, though on larger ships you might find enough space at least for modest point defense lasers or something along those lines.
As for the idea behind escorts, those are the ships that are designed for shooting down incoming missiles. Remember, even one missile shot down is one less that's tearing up your ships.
One last general thing: For warships, you'll generally have a fleet train of colliers/tankers/supply ships following them(generally hanging out on the other side of the hostile JP, of course), and they tend to spend most of their time orbiting friendly bases anyways, so you can generally get away with low deployment times. Having a deployment time of around 6-12 months is a good idea. As well, having enough gas for 75-100 days and a 3 year maintenance life is plenty(and may even be more than enough, but I like having it for extended operations). And finally, warships can be of any tonnage really, it just depends on how many weapons you can squeeze in, what role it is for, and if you feel, with the above two points, of it is effective in this role.
