So what's the absolute biggest tier difference pokemon can have on teams?

I like to, above all else, play with pokemon that I like. Problem is, I end up making mixed-tier teams because of this. I just ran with it for my first team and it seems to work, probably because it's just OU and UU which, as I understand it, are right next to each other.

But I thought, for fun on Showdown, I'd try building an all grass team just to have an excuse to plug a bunch of pokemon I like together, but the team I think would be the most balanced (insofar as a monotype team can be, anyway) has tiers all over the place from OU to NU so I'd like to know what the absolute largest tier margin can be for viable teams.

I'd also like to know a little more about tiers in general. Smogon tells me a little but I couldn't find where it tells me what PU is (beyond "doesn't fit in other tiers) or hat BL1, BL2, etc are.

Also, is there a monotype metagame I might demo teams in on showdown or would I have to go elsewhere for it, or just throwdown on showdown (hey that rhymes) and hope my team can hold? (and also that nobody is using ice types)

Well, tier list goes AG <- Uber <- OU <- BL <- UU <-BL2 <- RU <- NU
PU.. that's something newer that's composed of a lot of what were NU pokemon, so I'm assuming it's the NU pokemon that were used more.
BL3 and BL4 I have never even heard of, but apparently those exist. I'm assuming they'd go between RU and PU and PU and NU somewhere.

I like to use whatever pokemon I want too. Aurorus and Dragalge are fun, and they're NU and BL2. I'm convinced Aurorus is only so 'bad' due to having two x4 weaknesses.

PU stands for "p'ewww" as in pokemon in NU that have terrible stats (Sunflora, Heatmor etc) and PU is under NU in the tier ratings, so no NU mons in PU.

You can use whatever mons you like, but you have to battle in the tier of the highest ranked mon - so if you have one OU mon in a team of NUs you have to battle in OU. And remember the tiers DO NOT rank pokemon on effectiveness (except PU and Ubers), they rank them on usage. A mon with pretty ok stats and a good movepool can still be in a low tier because no one uses it

I did notice you are forced into the highest tier you have (good thing none of the pokemon I like are uber I guess) but an awful lot of them end up in PU...

I noticed some pokemon on smogon have different sets for different tiers though so it least it gives me options for trying to keep everyone as close as possible in tiers. I mostly just don't want to get completely destroyed and the first team I made on showdown was so mixed in tier that it lost a lot more often than it won, mostly becuse it only had 1 OU member, 1 UU member, and everything else was NU or PU sadly.

What do AG and the BLs stand for though? I thought BL might mean blacklist?

AG stands for Anything Goes, it's the home of things like Mega Rayquaza and Aegislash. BL is Borderline, it contains mons that would normally be in UU but are a bit more powerful and are at the border of OU (things like Hawlucha)

Well, tiers are supposed to be based on usage, but they do what they call 'suspect tests'. Even if Salamence has dropped in usage, they have deemed it too powerful for UU, therefore it lives in BL.
I honestly see all the BL tiers as useless anyway, since nobody specifically plays BL-inclusive tiers.