I have both an Ibanez MIKRO Rio (friends) and Yamaha TRBX504 bass. I use the full sized bass as I go through this course and practice in my dedicated bass area downstairs with my amp.
And I wonder if it is ok to practice with the lighter 3/4 bass when I am upstairs since it takes up less space and I can pretty much play it anywhere without distracting anyone, since I have a headphone amp.
Any thoughts on that?
Shouldn’t be much of an issue for you to adjust between playing the long scale TRBX504 and a short scale (Ibanez GSRM20?). Just a little bit of practice/time. No issue with doing the course on a short scale.
I don’t see the problem. You’ll run into fret spacing differences all the time. Your TRBX is a two octaves scale which has different fret spacing to the 21 or 22 fret Fender style bass anyways.
Plus going smaller and shorter requires very little adjustment, going from 34" scale to 35" you’ll need bigger adjustment and calibration.
I have a fair few basses (not as many as Al) each one is slightly different, one is short scale, one is a bass VI (Short scale, 6 strings, closely spaced).
It isn’t hard to slip between them, sometimes takes a few minutes tops.
Secret is to play each one often (and tell the wife they all play differently and are for different types of song…or…something)
No it doesn’t 
The fretboard is longer, but the spacing is identical. Scale length is the only thing that can change the spacing; the number of frets just changes how long the fretboard needs to be to add the higher frets.
Good point because I’m often switch from different scale length as well as brands and models so that could be my impression of the difference.
Then again it doesn’t bother me? 
Not really an issue. Of my 4 basses, I’ve got 2 short scales, a 4 string 34” scale, and a 5 string 34” scale. I don’t have an issue switching between them. Some work better for different songs.
I agree with this, in my (modest) experience the scale length is the same but on a variety of basses differing in neck width, body size and saddle position relative to the end of the body and where the strap buttons sit, they can all feel like they have differing scales, possibly due to where the hands naturally fall to.
No biggy though as you adjust pretty quickly
This is actually a thing. The way a bass sits on your body and the relative position of the bridge can make a huge difference because it moves the fretboard to the left or right depending on what you are used to. I ended up selling my second Warwick because of this in the end. Sometimes you just don’t like how they sit.
Yep, my Rogue Beatle bass is a 30” short scale, but because of the trapeze bridge coming so far into the body the nut is the same distance away from the bottom strap button as my 34” Stingray5. Along with the fretboard that doesn’t vary much in width, it’s definitely a different feel playing vs my 30” P bass. The BB is 35mm from outside of E string to outside of G string at the nut and 42mm at the bridge. The PB is 30mm at the nut to 58mm at the bridge. Quite a difference in plucking hand string spacing for sure.
as a beginner you almost certainly are going not by feel/muscle memory for finding your frets but more from simply using your eyes. so yeah, it would be pretty much not an issue for you imo. it gets slightly trickier when you are trying to use muscle memory but for me at least, it’s never really bothered me. but also i’m not a strict don’t ever look at your fretboard kind of player either, i try not to lock into it but i often take a peek or two. oh and shout out to the mikro btw, love them.
I get this with my acoustic. The bridge is probably 5 inches further forward than either of my electrics. I don’t find that a problem with the fretboard but my plucking hand always ends up almost on the bridge.
Maybe this is a minor counterpoint.. I have a mikro for the kids and primarily play a P-Bass. For while I would use the mikro to practice and it was “fine”.. I also found that between trying to learn a setlist I was doing too many tricks at once and I found that practicing only on the full scale helped me make more progress on the practice pieces.
I would say if you’re happy switching then go for it. If you find that switching between instruments is something that’s tripping you up, consider lugging the big one around.
Took me a bit of time to get used to full scale from a Mikro. But then after maybe less than a month, or even quicker because I never noticed, switching between the two became no issue.
For me I never had any issue swapping between scale lengths at all. It was a lot more about preference for feel.
But generally in my experience if you can play one you can play them all, skill wise.