I've never used one of those, but it should be no problem.
The down tube is not particularly thick, in fact, I think it gets thinner as you go upwards on it.
You may have to pre-heat the frame - especially at the SA pivot / peg plate if you are replacing the full cradle, or , if you are doing the basic lift the Y and insert a section, if you use a solid insert, pre-heating may be neccessary.
I've never pre-heated frames before welding, but I use a higher spec machine. Going apeshit with a torch, or an oven, basically defeats one of the key points of TIG, minimal HAZE zones. Especially with light tube fabrication that an AF represents. If it wont tack without pre-heating, you can simply use the welder to pre-heat, running along the junction, with the torch arc favoured to the section that is posing the problem - 1/8" tube should present no problem itself.
By the way, I've been TIG welding since my early teens, when I made my first frames - I'm now 51.
Different gases give different effects, to. Very marked effects, a bit of research will help you there. Here in OZ, I long since stopped stuffing around with gases other than our pure argon. You blokes use Helium, it's what you have in the US - no idea if that is better or worse than Argon.
As I said, I've no idea of the real capabilities of the 180 - it may be limited by its duty cycle, but, basically, if your doing your AF in the most common way, your only doing about 30" of weld bead, so it might not trip it.
Another limitation is the heat that the Much higher amperage will put into the torch and the line, assuming you've got an air cooled torch, but, once again, you are only doing short welds.
Get a buch of scrap, and practice, practice, practice, practice - then practice again. Try blatantly low amperages, and blatantly high amperages - and everything in between, it will give you a real feel for what is happening when you go to do the job itself.
Practice will show if you need to mess with pre-heating, due to your machine. It will also show you how little pre-heating you need even then, as the alloy pieces heat so quickly. Most of the time, the tack process gets all the heat you'd ever need in jobs like this.
If you have the foot control, it's much easier. You'd easily just be able to set the max amperage near the max adjustment, and 'lift off' the pedal as you weld. You'll be surprised at the amount you can / have to back down on the amps with alluminium, if you've only done relatively thick steel (as in 1/8' in steel, equal to that you'll probably be dealing with on the alloy cradle rails - 1/8 is thick to me in steel, as I deal with a some bicycle tubing that is as thin as .2 mm [.00788"] ). I'm starting to find that a bit difficult as my eyes sight and steadyness diminishes, sucks getting old. Though a strong Bourbon helps with those sort of fitments.
What you are welding is not hard to do at all, and you can position the frame easily to have great access to each weld, so no weird angles or approaches will come into it. Material prep, is of utmost importance. Clean, clean, clean joints.
'Tip Dipping' is something to avoid, and the rod needs to be held further away from the puddle, to avoid it just melting itself, but we are talking distance differences of mms here. And, as you've found, no 'colour change' at the weld puddle that you'll be so used to with ferrous metals.
Do you have the correct Tungstens?, the correct Collets and Cups?, The correct weld rods? Know the extension of the tungsten out of the cups? Check out Millers website, that, and Lincolns (which I think is even better) has aLot of welding tech advice you can look up. Check The Sites Out!
Alloy beads are much bigger, so that makes laying a bead easier, than minute, thin section steel /Ti beads.
Should be no problem, basically.