I have no problem at all switching back and forth between the ukulele and the bass. It does sometimes confuse me to switch between guitar and uke. Mostly it surprises me that improving my skill on the uke makes my bass playing better. You can safely learn many instruments without getting messed up.
When I played guitar I used a capo a lot so I got a lot of practice at looking at one key on paper and playing another. I also played saxophone for many years and had to transpose all the time. Learning multiple instruments and doing a lot of transposing will make you a better musician. I know people (I'm not one of them) who can pretty much play in any key on any song and barely miss a beat. I know that learning more instruments and transposing has caused me to learn more music theory which has made me hear things and understand things in new ways.
This is a subject of constant debate among woodwind players, kind of like which strings are better among ukulele players. If you play in a lot of bands, you are often expected to
play clarinet, saxophone and flute and to play all sizes of the sax. Flute is in C. Clarinet (mostly) Bb. Saxophone is Eb, Bb (mostly). And each requires a different lip and fingering. Pros are pretty much required to play all of them. There are some really great players who have decided to concentrate on just one (Eddie Daniels is famous for giving up sax for clarinet, early in his career Benny Goodman doubled on sax and clarinet). (I have found that at some point playing the sax will to some degree mess up your clarinet playing, but that clarinet playing tends to make your sax playing better. To meet the flute is neutral in its effect). They have claimed that playing more than one instrument limits their potential. But that has to do more with the lip and the different fingering techniques than it does with the key the music is in which is no issue at all. If I ever get as good on ukulele as Eddie Daniels is on clarinet, maybe I'll give up the bass. ( That will never happen.)