Ok,
If you use the MT400 You will need to mount it in your nosecone away from your deployment electronics. High Rf fields will dork your deployment altimeter.
I have seen charges deploy on the pad when the altimeter was turned on and I've seen two deployment altimeters lock up in a 16 foot tall 10" diameter project that then came in ballistic. Garmin dog trackers on the MURS band can have the same effect.
The 12mW Beeline trackers both NDB and APRS/GPS can ride next to the deployment electronics without a problem. Once one gets above 50mW output, I suggest you setup your rocket at home with bare ematches on the altimeter, turn on the Rf beacon and then the altimeter. Let the whole thing sit for awhile and see what happens. If the ematches fire, you have a problem. If the altimeter shuts down and the ready beep halts, you have a problem.
If using an MT400 as a beacon, you're right, you'd need a high capacity pack like 1000mah. Sooooooo, the MT400 setup one would likely want to save for a large project where you can mount it away from your altimeter(s). You could line the ebay with shielding, something like this:
https://www.goldmine-elec-products.com/prodinfo.asp?number=G17455
Or look up a Faraday cage to protect your altimeters. Kurt