I'm running a 5' RC behind my B7800. I have hundreds of hours with this combo in use: two different RCs. Have broken maybe three(?) shear bolts, and I've been fairly abusive. My NX5510 [HST] runs a 6' RC with a slip clutch. This combo would readily eat up shear pins- A LOT of mass and impact force.
OP, what are you cutting? Go with what's in the manual (grade) and you should be fine: LandPride, in particular, should be very reliable as to all this. IF you find that you're breaking too many shear pins THEN consider a slip clutch: either the LandPride documentation's mention of "either" is to cover the different models (usually manuals cover a range of equipment) or it actually is an option- if it's an option then you might fall into the reason for that option (and go that route!).
NX stalled out when my 7' flail choked on a chunk of 8" (unintentionally hit). Quite relieved to find that nothing was broken (except my nerves). The flail (case for many) doesn't use a slip clutch or a shear pin- in general, flails tend to rely on belt slippage. My flail has four belts, which I figure might make it tough to slip. Gonna have to ponder all of this with this setup...