The big thing I'll add is, quite simply do not be afraid to retreat. This game, much like the tabletop and other tabletop games like it (by which I mean skirmish games where units carry over) rewards long term play over short term gains. Losing a ship or two in a battle is managable, losing the whole fleet is positivly punishing. While destroyed ships do not get destroyed forever (well technically they do in that their # battles cosmetic stat resets and they get a new name), retaining their upgrades, you must wait a few turns to get them back, which is either turns on the solo play campaign, or matches in MP. This can leave you crippled if most of your high level ships are dead, or possibly in a dead state if every Line Ship is dead, althouh this shouldn't happen as I can't think of any point value that would let you field every ship you own unless you've been refusing new ship tiers. But yeah, keep that in mind. It's much like Blood Bowl or Mordheim in that respect TBH, where at some point you need to minimize losses.
Also one of the biggest decisions in game I find is to attempt to stop a warping ship or not. Once a ship retreats to the warp that's it, it's gone for the match, BUT, you get less XP for a heavily damaged ship than a destroyed one, and if it retreats before even reaching that state you get no bonus XP. So it's a risk reward of going for the XP vs keeping a combat unit on the field, especially as synergetic buffs from allies could help revitalize it for a short while, or it could be used in a suicide ram.
As to the boarding confusion, there are two types of boarding. Lightning Strikes and Boarding Craft. I'm not entirely sur eon the exact differences, but Boarding is far more devastating, and really the only time you use Lightning (as they share a cooldown) is when you dont think you can close the gap to do it proper, have the Adaptus Astartes favor for an Imperial ship, or need to stop a warping ship from escaping.
EDIT: Also, I'm not 100% sure but I think so, having a ship destroyed resets the xp progress that's its gained twords it's next level, which can be quite painful if you were say 400xp to a 450xp levelup.