I finished the game with 265 goo. I see some people with 800+. I don't see what levels I could have got that many more goo on. What are some good levels to redo to get more goo? Are there any secrets? bonuses?

There aren't any tricks or secrets to it. Just keep trying levels and you'll keep getting better. The maximum number of usable goo in the tower is 300, so you don't need many more. Keep working at it.

There are a couple of tricks to try.  I'll try to avoid spoilers here....

If you haven't completed Leap Hole in Chapter 2 yet (you can win the game without doing so, I think), go back and figure it out.  It's one of those obvious-in-retrospect things, and it can be done in any level with detatchable Goo, which should get you a couple dozen extra in a few levels.

Beyond that, there are a few levels with very high Goo counts to collect, but usually those levels provide even more extra.  Replaying those can be very lucrative.

All else fails, check out the OCD videos, they're full of nifty tricks.  It's really not that hard to get to 300, keep at it.  If you've finished all the levels (including the Epilogue) you can probably handle this with just a little more effort.

I figured out how to get a whole bunch more on Welcoming unit. Now I have my 300 and I'm up to 26.3 meters. I think my tower needs a redesign to get any higher.