I would recommend just sticking with one or the other (tourney-style, or cash tables.... although deepstack play on cash tables will be beneficial to playing tournaments (deepstack)... it's probably best to just stick with one or the other when you're getting started).
I guess I could tell you what I would do if i had it to do all over again. I would start off with 9-plyr. SNG's & large-field MTT's. Since alot of your current experience comes from playing in freeroll tournaments, I don't see a problem with playing micro buyin MTT's.
With SNG's I'd stick to either single table or mult-table but I personally would just play the single table games for awhile. Read up all you can on playing SNG's as there's some basic key principles to playing them.
If you also choose to play the 18-plyr. ones, know that you'll be playing shorthanded before the two tables join and you must loosen up your starting hand requirments a bit in order to survive (or look for spots to resteal form players who are playing too many haneds).. reason being, you must make it a goal to get to the other table with ~2,500chips min. (hopefully) so you won't be in dire straits on the final table when they do combine.
If I had to do it all over again.... I would play SNG's for ~300-500games and then consider playing MTT satellites (one's paying ~1in4 or 1 in 5)... these also have a very specific, straightforward strategy for playing them and one can capitalize on those who are frequently making mistakes in them.
Then I would add in more MTT's.
Then I would consider getting knowledgeable about cash game play and would start off at the lowest limits first, putting in ~5,000-10,000 hands prior to moving up. Full ring would be probably easier to start out with.
I would focus on one or the other but not both at the same time (tourney-style (ie. SNG's & MTT's) or cash game play (full ring or 6max).
Find what you like the most as this is where you'll be motivated to put in the time to study the game (obv). Personally I prefer MTT's over cash game play but many are the exact opposite and might suggest that you get a solid micro limit cash game going prior to playing other stuff. Whatever suits you.