It depends on your role and game mode: PvE: tank = Ion Gas Cylinder dps = Combustible Gas Cylinder PvP: In most cases run with Ion Gas Cylinder so you can Guard a healer. If you are running solo and no good healers, you can go with either one.

Heat Blast is your heat dump. Flame Burst is your dmg reduction debuff ability, assuming you took Combust.

You should be able to go back to Taris and complete the quests. You just may have to do a bit more hunting to pickup the questline. One bonus, since you outlevel the instance you should be able to solo everything.

If you do all the bonus quests in Taris, you can get the Powertech Elite (or dps equiv) set. http://www.torhead.com/search/powertech%20elite%20upgrade