I've used Bionicle joints for a Bruticus head which is pretty large, so I would guess it will be fine for Leader Megs. As for the arm, the trick is blending the Bionicle joint into the new, less-gimpy arm...maybe your best bet is glue TF/armor-like parts onto the lego arm of correct length rather than attach/blend it into the TF parts.
Either way you will find those joints sturdy enough for arms, provided they are not entirely made of putty.
--Echo