Ductless mini-split options - help!

I have decided on a ductless heat pump mini-split system for my house and I’m thrilled. 
I’m keeping, for now, the very old gas gravity floor heater which heats the front part of my house. 
What I’m adding is a Mitsubishi 3-zone system to heat/cool the three other rooms in my split-level house. (Bedrooms, office). 

The two downstairs rooms will get square registers (cassettes) in the center of the ceilings. The upstairs split-level bedroom will get a wall mounted unit. Each zone heat pump will be 9k BTUs. 
I’ve gotten quotes for a 24 and 30 BTU outdoor condenser system. 
Any pros and cons to the condenser size? Prices, footprint, and sound aren’t that different. I don’t want to oversize and waste energy. 
About the controllers: most folks get “remotes” which generally I don’t care for. I dont want to be dependent on an app for temp control, either. I’d like a hard-wired, walk-mounted, programmable thermostat with override and on/off capability like I have for my gas heater. Has anyone used this kind of control? 
Also, they offer special filters which sounds great as I want indoor protection during future smoke events. 
Any feedback and personal experiences would be so appreciated!

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I don’t have any answers to your questions, but want to caution you to consider your utility bill impact of combining gas and electric heat- you may want to try to talk to someone at PG&E before finalizing any decisions. California energy policy mandates tiered usage rates where the more energy you use, the more higher the electric rate. It’s meant to incentivize energy efficiency/conservation. But of course switching from gas to electric heat will increase your electric usage but be an overall climate win, so when you switch to electric heat, you can tell PG&E and they’ll raise the “baseline” of how much energy you can use in the cheapest tier. I don’t know how it works if you still have some gas heat in your home, though. You may not be eligible for the higher baseline and may end up paying much heftier electric bills, so it’s worth looking into.

You are smart to keep the gravity furnace, and to augment it for additional needs. It requires no electricity at all, not even for the thermostat, so you'll enjoy a heated home even when the power is out. And it's completely silent. We had one in our previous house -- it was 80 years old, completely maintenance free, and bullet proof. So many have been ripped out by people who didn't understand what they had. I wish our new home had one.