If you want me to take a deeper dive into your suggestion, I'll do it. Here we go.
1. Removing fire delay on flamer would pretty much remove the only opening xeno have to engage you with, since you can just spray and pray to prevent most xeno from doing shit. I know that it seems like fire doesn't do much to xeno, but that's why you need to play xeno and see how cancerous it is. It takes 3-4 resists with each of them stunning you for 3-4 seconds. Just that alone means that the xeno will take a minimum of 9 second stun and a maximum of 16 second stun. Fire also removes queen healing, as said above, so the xeno immediately has to run back and shakes or has the fire put out or it will die to every other weapon without having the safety net of queen heal.
2. Queen and Rav fire immunity. It's 100% needed due to the fact that just spraying fire in a path to block xeno is so braindeadly effective, that there needs to be a counter to it. If anything, if flamer has a bigger fire delay and you don't need to stun yourself for 9-12 seconds to remove the fire, they wouldn't need to have fire immunity.
3. I already mentioned above. Mini pyro kit comes with extinguisher already, and on the Almayer alone you can find 30 something extinguishers to outfit marines with. All of these extinguishers can be refilled with a water tank you can find either on the ship or on the planet. Xeno only has 3 castes, 1 being T2 that needs a strain for it (the strain is a pure downgrade); and the other 2 being T3s. Xeno will need to have 50 xeno to outfit 15 T3s, and all of those 15 T3s need to be boiler or base/RG prae to even compete with marines in terms of extinguishing fire. Extinguisher can put out a 3x2 radius (6 tiles), while boiler can only do 1x6 (6 tiles) and prae can only do 1x5 (5 tiles) or a cone of 1-3-5 (9 tiles) and vomitter spitter can only do 1x4 (4 tiles). But, here's the thing. If you miss your extinguisher spray the first time, you can just spray again with little cooldown, while the boiler/vomitter/prae needs to wait 10-15 seconds to be able to spray again.
Real problem: Fire is too easy to just spray out there and stop any combat from happening, for both marines and xeno. It stops weed healing, queen healing. It takes 9-12 seconds to shake the fire off (unless it's LV river); or 1 of 3 castes
Real solution: Make resist takes less time, and remove the RNG in the amount and time needed for resist to shake off the fire. Then you can buff fire damage or lessen the delay. Also, reduce the availability of flamer. Old flamer can instakill most T1s (braindead gameplay, but still has the same counter as current flamer), but you can only get 2 of them per SL and that's pretty much it.