I'm familiar with both the banjo and the ukulele, but not that much with the banjolele, so I can't give a definite answer, namely about the structure of the instrument being prepared for the higher tension of steel strings. However:
- From what I have seen in instruments being sold and played, "nylon" type strings are normally used.
- Normal ukulele techniques, specially the most difficult ones, will be more difficult to play, if at all possible, with steel strings, due to the higher tension.
- The sonority will most certainly be quite different between the two types of strings, but that's a matter of taste. The normal sound of the banjolele (with nylon type strings) seems to me quite similar with the uke, with just a bit more resonance and volume. Perhaps steel strings, if viable, can be an interesting experiment creating a more different sound.
- Can the banjo strings be fit the banjolele?
- Let's take 22" (56 cm) as a typical banjo scale length, and 15"3/4 (37,5cm) that of the concert banjolele. Then for the same working tension of the strings, the lenght proportion of the strings is that of 37,5/56=0,67 or almost exactly 2/3.
- So the note produce with the banjolele will be a interval of a 5th above the note in the banjo with the same type of string at the same aproximate tension.
- So the first (thinner) steel strings of the set you got will give the same tuning as the nylon strings. But the two thicker strings will give much lower pitches thatn required for the banjolele and to tune them up a 4th a 5th above will amost for sure stretch the strings too much, either breaking them, or at least making them extremely hard to play and putting the arm in great tension.
If you're keen on experimenting you could try to find strings with lower gauges (even guitar strings) and try to fit them (I've done that with cheap ukeleles). But all in all, and with the caveat that I don't have hands on experience with the banjolele, I would advice against using steel strings.
Regarding tuning, the normal (or at least the more commont) tuning for most ukelele sizes (with the exception of baritone) is G-C-E-A. I expect your banjolele manufacturers consider that the default tuning for the instrument. However they are telling the user that the instrument can also acommodate a D-G-B-E tuning (a 4th below), giving it a sound closer to the baritone ukulele, either by loosening the strings (they will probably become rather slack) or (better) with higher gauge strings like the baritone strings you purchased.