You are between a rock and a hard spot and now your heading for a cliff...
SLOW DOWN
If it took ten days to raise the ammonia in a 55 gal tank ...the same Shark will raise the ammonia in two days in a 10 gal tank...That's not long enough for your main to cycle/stabalize/change...Now your Shark would be out of the frying pan and into the fire...
In other words it would be MORE stressful to move your Shark into a smaller tank that is inherently LESS stable than a larger tank.
Use the Five gal of aged/aerated water for a water change in the 55...Dillution is the Solution to Pollution...
Amquell will convert the toxic Ammonia to less toxic ammoniUm...(still bad stuff) and not affect the cycle process...
You still have NitrItes to deal with...
"Cycle" or "Prime" is additional beneficial bacteria (like the kind on LR/LS that does the converting, without the additional bio load of "die off from additional LR) Which will speed up the process of multiplying beneficial bacteria, helping cycle along.
Continue to test and do small/continuous water changes to keep levels down...This will
stretch out your cycle for a longer time and only give your Shark "tolerable" conditions rather than healthy or pleasant living conditions.
It's going to be alot of work, take longer than it should, will be STRESSFUL to your Shark, and MAY NOT work anyway...
I hope you can see the importance of convincing the LFS to hold your Shark for you while your tank cycles in a more 'Normal" fashion.
They may be worried about a "Guarantee" to keep your Shark alive...Tell them you Don't need a guarantee because you know the Shark has a better chance with them than in a "Cycling" tank.