The laws of war at that time firmly condemned military activity by unregulated and unauthorised bodies, even militia formed to protect a town. In theory no civilised general could have any support for such bodies and those taken in arms could be hanged or shot. It is not surprising that British generals were suspicious of anyone getting involved with Calabrian peasants, especially since their bloodthirsty tendencies had been luridly reported in 1799.
In Spain there was simply no way of avoiding them, I doubt if Wellington would have had a hand in setting them up but, being a pragmatist, he would not have turned his nose up at their services. The matter was complicated by Soult's edict that since there was only one official Spanish army, that of King Joseph, all others were unauthorised and therefore counted as brigands, no matter how many generals they had and how good their uniform.

regards,

Susan