Hi,
I have sims 3 pets for the Ps3... So they are pretty identical. You cannot make a separate garden on another lot. The fire meter is a huge annoying issue, but shouldn't affect your sim playing too much.
The bigger your lot, the higher the meter = the more things you can have. My tip is to move to a big house, I just moved to the $140,000 Gilman mansion through no cheats, just hard work. The fire meter is much bigger than if you expand on a smaller house, like the cheaper ones.
Move to a bigger house and do a rotation system. Plants only last for a few harvests, so, for instance, plant five plants at a time. This also makes gardening a less time consuming hobby because, believe me, tending twenty plants a day takes about ten sim hours and is impossible with work.
Try this, or have less things in your home. Please reply if you need any extra help!