The Inverter model numbers you mentioned are Off-Grid type and do not feed back to the grid. When you say "Tied-In" to the Main Panel are you referring to having the Inverter AC Output feed the Main Panel? If so then you would need to disconnect the utility power from the Main Breaker, add a new main breaker (2 new Main Beakers for 2 inverters) and feed utility power into the Inverter AC Input.
The other way the inverter(s) can be connected is by adding a Critical Loads panel that is fed from the inverter AC Output then you could add a new 2 pole breaker in the main panel to fed the AC Input of the inverter. You would have to move all the circuits you want on back-up power to the new Critical Loads panel. In either case, since the inverter cannot push power backwards out through the AC Input there is no grid feedback.
Back to your other questions.
Many of the All-In-One Inverters (MPP Solar, EG4, Growatt, SunGold) are stackable so you can definitely add additional kW output later. However, most manufacturers will eventually discontinue some models and introduce new ones and the new ones probably will not have compatible firmware so don't wait too long. Also, many models individually only output 120V unless you have 2 units and set them up as 240/120V split phase. I would advise starting with a single unit that is already able to supply split phase then adding another one in parallel is fairly easy. Note that the maximum DC input voltage from your PV array can vary by brand and model number from as low as 150V up to 500V. Higher voltage is better because it allows you to string more panels together in series rather than several parallel strings which require combiner boxes, breakers and more wiring.
Think ahead in your layout and have spot picked out for the second inverter. Size the breaker panel for what you want to end up with not where you are starting so later you don't have to replace it with a bigger one and rewire everything. It doesn't cost that much more for a 150A panel with 30 spaces vs. a 100A panel with 20 spaces. When the second inverter is added you will need a combiner/distribution box where the 2 outputs are combined into a single larger wire feeding the panel. If your budget allows, install that in the beginning also. When the second inverter goes in all you have to do is wire it up using gear that is already in place.