Roseline shark and red tail shark together?

[MultipleTankSyndrome]

For what Fishman Dave said, that was a small error on my part. I forgot to put in 'most likely', since of course there are no guarantees.
I think your tip about having 2 red tails, provided 1 doesn't get overly dominant, is a good one. Since, as you point out, their main concern is each other.
If having a pair is tried and one gets dominant, it will be returned while bearing in mind the less aggressive one will be more likely to leave well enough alone when it comes to its tankmates.
Alternatively, one way to select a more peaceful red tail is to choose one being picked on by the others in the store tank but is still relatively healthy. Should that (or the scenario outlined above) fail, the previously mentioned plan of moving them to the other 473 will be carried out, and if that still doesn't work they can ultimately be returned.

I appreciate the suggestion made by Stephen St. Clair, but red tail loaches are too aggressive for yoyo, zebra, and Burmese loaches (plus they get large and are nocturnal).

 

[A201]

The Red Tail Botias certainly do get big & thick. My group are active day & night. They don't bother the YoYo Loaches or Tiger Loach Tankmates.
Other than being a little over boisterous. I doubt they would go after more timid Botias. Maybe out compete them for food.
I've seen an XL Red Tail Shark return / donationat my LFS. Quite an impressive fish.

 

[celebrist]

They are beautiful, mine was 4 inches or so and 2 or 3 years old

 

[cockroach]

For what Fishman Dave said, that was a small error on my part. I forgot to put in 'most likely', since of course there are no guarantees.
I think your tip about having 2 red tails, provided 1 doesn't get overly dominant, is a good one. Since, as you point out, their main concern is each other.
If having a pair is tried and one gets dominant, it will be returned while bearing in mind the less aggressive one will be more likely to leave well enough alone when it comes to its tankmates.
Alternatively, one way to select a more peaceful red tail is to choose one being picked on by the others in the store tank but is still relatively healthy. Should that (or the scenario outlined above) fail, the previously mentioned plan of moving them to the other 473 will be carried out, and if that still doesn't work they can ultimately be returned.

I appreciate the suggestion made by Stephen St. Clair, but red tail loaches are too aggressive for yoyo, zebra, and Burmese loaches (plus they get large and are nocturnal).

Every fish has it's own temperament and could work in a setting not usually suggested.
Having said that, I cannot overstate what a bad idea I (personally) think this is. 1 Red Tail Black Shark can be aggressive, 2 is caged hunting for one of them.

The aggression in these fish often stems from improperly scaped tanks or mostly tanks that are too small and the fish feels it constantly has invasions and needs peace. The minimum tank size stated on almost all websites, such 29gal or even 40gal, should be considered minimum size if you know what you are doing with a territorially aggressive cyprinid. A 4' (55~75gal) tank is a way more appropriate fit for these fish with the latter negating a lot of the "aggression". They can chase a fish that can get up to 3' away from it so it loses interest. It more importantly, has the room to move away from fish and not feel intruded upon.

You could do a large group 12~15 so a fish is not singled out but you would need a huge tank for that to work and a very carefully, biotypically scaped tank to accommodate that much 'tude.

After all that, roseline sharks don't look too similar, occupy a different area of the water column (mostly) and are really fats in their own right. This trifactor should see them sitting pretty with a red tailed black shark in an appropriately sized tank. With added water movement in a confined space they seem to settle even more.

Mine is housed in a 75gal, with HIGH flow rate and water movement. It is housed with clown and yoyo loaches, rhinogobius and sicypoterus as well as anostomus and leporinus. He is happy and mostly chilled. Occasionally chasing a fish a few inches and that really does seem more like it is "for the hell of it".