Just as a general tip for Cross breeding, you'll want to have empty tiles next to your crops with both tiers of crop-sticks on them (Aka right click on that tile twice with crop-sticks, or shift-click).
To better the chance of cross-breeding, you want to keep the two plants in the 'mature' growth stage so try not to harvest them. You can bone-meal them to speed this up (unless they're essence crops, in which case you'll either to wait or use Magical Fertilizer).
Once you've got yourself some Iron, you can create some Garden shears which let you cut the top off of mature plants and place them down on other tiles like seeds. However they don't always succeed, but the higher the strength of the original crop, the more likely they are to succeed.
The other two stats boosts the growth speed and the no. of drops from the plant. Hence why the most avid of gardeners try to shoot 10/10/10 crops. Which clearly isn't me, because most of mine are not that perfect
And to answer your other question Pook, you don't have to! But if you are playing alongside someone else then it is certainly helpful! The benefits of being in a party are, when someone completes a quest, it's completed for the entire party and each and every member can claim the reward. Even if your offline at the time, you can still claim the rewards when you log back on.