Congrats on completing Fish Tycoon! I would have by now if it wasn't for my previous inexperience with handhelds, and my incessant need to tinker with anything and everything technological that I can lay my hands on
As for "cross-breeding" plants, think more along the lines of "cross-pollination"
Yep, you just drag the pollen from one plant, to the one you want to cross it with. Just in case, I'll mention that the little icon for pollen is in the lower-right of the main greenhouse screen, between the seed icon, and the price icon ^_^ Tap and drag the pollen from that button directly to the second plant
I hope your luck with the seedlings is a good as your luck with the fish! ^_^ Have fun!
- Kate ^_^