Hi Roger, you can go with a single phase inverter. It really depends on what you have connected on each phase as to what will work better I believe. If unsure, it would be best to discuss with an electrician, preferably you should get an electrician to quote your solar and they should be able to assess if best to use 3 phase or single phase inverter.
You can oversize the inverter by 30%, and there’s often good reason to do so. Fronius does a 3.5kW inverter which is probably better suited, or a 4kW inverter Is fine on a perfect nth facing roof with no shade. There’s some details here in the Guide To Going Solar eBook on oversizing the inverter:
https://forum.cleanenergyreviews.info/t/ch-9-how-big-a-system-should-i-get/162
I would strongly recommend making a decision now about whether you want 6kW or 4. If in doubt, go for 6 I think, but no, I wouldn’t recommend getting a 5kW inverter to upgrade later, it just gets messy. Plus labour cost is the big killer, getting it all done in one hit will be far cheaper.
Hope that helps.
Marty