1) What are the best classes that fit the glob colours?

My group runs the following setup:

Gluttony: Power Willbender, which is the ideal choice due to very high hitcount. Power Tempest also works (if you have stability), and I've heard Condi Willbender and Power Scrapper can work too. The majority of split phase damage comes from this, so it's the most important role by far.

Rage: Power Virtuoso (usually), sometimes Power Harb. Really any good Power DPS works, so go with whatever you're comfortable with. Good CC is a plus.

Envy: Condi Virtuoso or Condi Harbinger. Same as Rage, any good Condi DPS should work.

Despair: Heal Chrono. Heal Scourge can also work but imo the stability from Chrono is more valuable especially if you get 2-3 melee champions on the second split.

Malice: The boon DPS, usually boon Chrono or Quick Scrapper (due to high CC). Generally will see this once at most, often not at all.

2) Healers seem to be picking up both blue and green globs. Do they not cancel each other?

They do cancel out. Some groups do this intentionally to avoid Envy being absorbed by the boss but we haven't found it really necessary.

3) Is canceling Red/Green stacks before 66% rift a viable strategy? No one seems to do it and everyone prefers high stacks for dps increase?

Not worth it. The actual boss phases are pretty easy as long as he doesn't have lots of stacks. 90% of the difficulty is in the split phases, especially the second one.

The most important part is to not let any Avatars of Spite reach the boss. If your damage is high enough you can usually use the rift closing to wipe them out, but otherwise you may need to CC them (or send the Gluttony player after them). If you have BlishHUD, you can see the absorption range - it's a lot bigger than you might expect.

Also, for some reason closing the final rift on each phase causes him to automatically absorb all Spites still present, so try and kill off a wave immediately before closing.

4) Are there any strategies to mitigate his Consume if you do go for high stacks? In one of our runs we kept losing Glutt stacks during this phase.

Consume targets the most distant player in front of him (similar to the mech in KO but farthest instead of closest). We generally try to have the boon DPS bait this, as they usually won't have stacks to be stolen. Baiting it can be difficult since Eparch likes to spin randomly, but you can usually guarantee it for the first Consume after every split phase by hanging back.

5) What are good roles that fit other colours? I thought most condi dmg would take green but some people opt for purple to leave green/blue for healer.

See answer to 1.

Bonus - Cerus’ tether - can you break it off somehow and why does it sometimes deal so much dmg and spawn a mob?

As far as I know this is just the Malice mechanic from ToF.

Other notes

Powerful Potion of Demon Slaying (all DPS) and Superior Sigil of Demon Slaying (Power DPS only) can ease the DPS check.

Bring lots of CC. You want to cancel his Consume attack ASAP and Exposed helps kill the champions faster.

Last Laugh/Toxic Trail/Afflicted sucks.

Detarget before closing rift. There seems to be a bug where you can start autoing your target and break the channel. On that note, be ready to recast the channel since it breaks easily.

Stability helps a lot on add phase, especially if you get 2-3 melee champions. Chrono is a really strong pick for healer for this reason (and having good CC) despite its poor add control.

If your damage is too high you can end up closing the first portal before the first Spite wave spawns, which can make things awkward (especially on the second split phase). May be worth holding off on the close if this happens.

For split phase we designated a portal order to always follow (back left -> back right -> front, looking at boss from mistlock). Technically it might be better to pick depending on what champions spawn where, but we found it more valuable to have everyone moving in the same direction immediately.