4x6550 will draw 6.4A of heater current (1.6A per tube), before you even consider anything else like preamp tubes (another 1.8A). This is more than a Twin PT is rated for - it will fry. If he doubles his speaker load and fits 2x6550 the PT will handle that OK. The amp's dynamics will be improved as will fidelity, outright power (W) will remain about the same but I can't really imagine a scenario where this wouldn't be loud enough.
You need to replace the 470ohm screen grid resistors with 1K @ 5W.
Replace the 1N4007 diodes with 4x1N4508.
6550 might be happy at 40-60mA per tube depending on plate voltage.
To determine PT ratings decide on:
1. what your B+ will be for the B+ secondary voltage - E.g. 350-0-350VAC will usually give around 480vdc when rectified with SS, loaded.
2. work out how much B+ current you will be drawing from the B+ rail for the B+ current rating - plate voltage per tube, plus a margin for a rise under load in fixed bias (25-30%?), 10mA per power tube screen, 10mA per preamp tube. For 4x6550 that will be somewhere the wrong side of 400mA, but for 2x6550 it will be 240mA-ish and a stock Twin PT will handle this.
3. Add up how much heater current draw you will get from the tubes installed. For an amp that runs 4x6550 & 6x12A#7 that's 8.2A minimum (to be safe double it, don't even attempt this with less than 10A) - I doubt that you will find such a PT that will even fit in your chassis. Again, for 2x6550 in a Twin, current draw will be the same as stock (4x6L6), no problem.
I had a Twin once, fitted with 2x6550 and 8ohm speaker load (2x12" & 2x10" - it's what I had handy) it was the most fantastic heavy, clean sound I've ever heard (12AX7 in the BF PI, dropped preamp voltages (170-180v on V1/2 plates, 2.7K cathode resistor at 1st stage.)