"Do compound lifts only" vs "Do isolation only": They both have their place. I like seeing compound exercises as the basis that allows me to work on strength and fatigues a lot of muscle fibers across multiple muscle groups. I then add isolation to target the specific muscles. If I have the time and the energy I hit all the main muscles I worked during compounds (my split is essentially upper/lower/lats+biceps), if not I focus on what I consider to be the most important.
I don't try to separate compound and isolation exercises as much as some people seem to do. They're both used for strength AND for size. If I suddenly switched to doing isolation ONLY I imagine I would have a hard time fatiguing all the muscles sufficiently and in a safe way. On the other hand, if I only did compound exercises I would not be growing proportionally.
Whether you put on a lot of fat or not depends on a lot of factors. If I only train 3 times a week and are barely physically active beyond that I gain a lot more fat than muscle. That's what happened to me the first time I "bulked" a year and a half ago and led to me being much lighter than I'd like to be at this point. Now I'm training 6 times (4 heavy sessions and 2 light ones) and walking a lot. I'm starting to add more calories in now (as in actually started this week) and I'm going to see if I actually have any idea what the hell I'm talking about over the next couple of months.