1. But that has judgmental overtones, so I'll hold that in reserve.
但这个带有批评色彩，还是作为备用吧。

in reserve
备用

e.g: We always keep some money in reserve, just in case.
我们总是存些钱备用，以防万一。

2. Next to you, I'll look like a catch.
 
有你在旁边，我更有把握了。

a (good) catch
可托付终身的好人（幽默）

3. I'll have to have you over for spaghetti some night.
我改天晚上叫你过来吃。

have sb. over
请某人到家里来吃饭

e.g: We must have you over for dinner before we leave.
在我们走之前一定要请你到家里搓一顿。

4. You can't hang out with your roommate's ex.
你不能和室友的前任一起玩。

hang out with
与...出去完；与...闲逛

e.g: I don't really know who she hangs out with.
我真不知道她和谁在一起玩。