It sounds like your LBS is retarded. The Supercharger bash guard comes in several sizes, this is taken from e13's website:
4-104 BCD in 32T, 36T, 40T, 42T & 44T sizes
5-110 BCD in 36T, 40T, 42T & 44T sizes
They may have gotten you one of the 40-something tooth sizes which are for DH bikes running big rings. The 32T will cover stock middle rings, but I would get the 36T in case you want to change middle rings later. It will still be smaller in diameter than an outer chain ring. And no, you would not have to remove the middle ring in any case to install a bashguard because it only fits in place of the outer ring, the fact that they even suggested that would be enough to not want to go there again. Also, for future reference there is really nothing to installing a bashguard you just unbolt the outer ring, slide it off over the crankarm (no crankarm removal necessary), and bolt on the new bashguard.
4-104 BCD in 32T, 36T, 40T, 42T & 44T sizes
5-110 BCD in 36T, 40T, 42T & 44T sizes
They may have gotten you one of the 40-something tooth sizes which are for DH bikes running big rings. The 32T will cover stock middle rings, but I would get the 36T in case you want to change middle rings later. It will still be smaller in diameter than an outer chain ring. And no, you would not have to remove the middle ring in any case to install a bashguard because it only fits in place of the outer ring, the fact that they even suggested that would be enough to not want to go there again. Also, for future reference there is really nothing to installing a bashguard you just unbolt the outer ring, slide it off over the crankarm (no crankarm removal necessary), and bolt on the new bashguard.
