Run a Shimano STX derailleur with 8 speed cassette or bigger? I've heard that it's possible to run the same derailleur with different shifters to get a different number of gears. I would like to know what the limits are to this idea.  I want to keep my Shimano STX rd MC30 rear derailleur because I like the smokey chrome color but I want a bigger gear range in the back to run a 1x.  I am currently using a 7 speed shifter.

I understand that this upgrade will require a new hub to fit a bigger cassette.

Thank you!
Basically all the &lt;=9 speed Shimano rear derailleurs are compatible with the &lt;=9 speed Shimano shifters (excluding some old Dura-ace and a few other weird exceptions). So, you need to match:

1) Being within the capacities of the derailleur (max tooth, min tooth, chain capacity -- based on the chainring and cog sizes)

2) # of speeds on the shifter = # of cogs on the cassette

As for needing a bigger hub, check that your hub is indeed a 7 speed hub; if so, you need a new hub (which a new wheel would likely be the upgrade route), but a lot of people have 7 speed cassettes on 8/9 speed hubs. 

Note that the cost of the new wheel may make the upgrade route not worth it economically (you're looking at the cost of a new shifter, new cassette, new chain and a new wheel). 

Based on the derailleur, the bike is probably close to 24 years old at this point, which makes it questionable to do these upgrades at all (and likely that you have an actual 7 speed cassette wheel, or maybe even a freewheel based bike, which would require an 8 speed freewheel or a new cassette wheel). I'd advise getting a newer bike from the get go instead and leaving this the way it is, if you want to do a 1x8 conversion.

Also, there are other smokey grey derailleurs on the market from Shimano (e.g. in the Acera line).