Hi, I recently got an 80W Philips T5 fluorescent lamp, but i only have a 35W HE ballast. I've been wondering if there's actually a difference between these lamps like different gas fill or they are just compatible with other ballasts because the lamps are the same length.

If I remember right, HE T5 tubes run at ~175mA current like all historical T5 tubes back from 1940's F8T5, having low current density for max efficiency, while HO's are driven much harder at up to 430mA, to pull max lumens per unit. HO electrodes shall be made much beefier to run in optimal thermal mode at high current. That means if you run HO tube on HE ballast it will end up running with cold electrodes, being killed off rather quickly.

 

There are special ballasts for both HO and HE, or only one. Here, some ballasts are HO and HE compatible (same length different wattage) while some are HE only.
For example, a ballast which supports both 54w and 28w and a ballast that only supports 28w.

Hi, i recently got an 80W Philips T5 fluorescent lamp, but i only have a 35W HE ballast.

Try it, it may work just fine (and no risk of damaging anything):

I've ran 54w lamps on 28w ballasts before (which is doing the same thing, just with a shorter lamp)...
And it all depends on the ballast, from what I've found most will light/run HO's just fine.
Sometimes you'll get one that just blinks/trips the ballast's EOL protection(basically it detects an incorrect or bad lamp & shuts off as a safety feature)

May be not the best advice, as if you google GE specsheet for T5 tubes (rather detailed!) you will see 80W tube is rated at 0.555A (!!) and 35W one for just 0.175A

In practice, fluorescent tubes are said to survive being de-rated by current about 50%, (without extra pre-heating) going further you are asking for big troubles.

Like xmaslightguy, I have run 54w HO lamps (nominal 4 foot) on 28W HE ballasts. In this case, the ballast was electronic program-start type, so cathodes are preheated during startup, but I'm not sure that they remain heated during operation; my guess is they don't. I found I got reasonable lamp life (maybe 5 years or so? I can't remember but it was used a lot) nonetheless, and the ballast didn't seem to get hot or have any issues. Running the HO lamp at much lower current didn't double its life, though.

I'd not advise the other way around (HE on an HO ballast) as the lamp isn't designed for higher current and would likely suffer significantly shorter life.

Oh, I'm also running some Powergroove VHO lamps (1600mA) on an HO (800mA) ballast and so far they are doing fine as well. I have some defective lamps that will blow out (slowly lose vacuum and die) on their proper ballast at 1600mA but they seem to survive when run on lower current. I believe these ballasts have continuous cathode heating though unlike the T5s.

Practically all electronic ballasts keep technically feeding the filaments with a current, but it provides negligible heating power compare to what the discharge root does (mainly the cathode fall of about 10..15V at the arc current, split among both ends). It is more an acceptable consequence of the way how the circuit is designed, rather than really "sought-after" feature.
So when operating 555mA lamp on 175mA, the cathode fall heating would become 30%. To maintain the temperature, the cathode will operate at way higher cathode drop voltage to compensate and that would cause more energetic ion bombardment so way faster wear.
Plus the preheating power designed for a 175mA lamp won't be sufficient for the 550mA on at all (the circuit uses to be designed as a voltage source with reactive ballasting so around the heater rating the power is rather independent on the heater resistance; and the 550mA needs about 3x higher power than the 175mA one), so even when the ballast is of a "programmed start" type, it won't be effective, so the lamp starts would wear it practically the same way as an instant start.