When checking my own three- and four-part modal exercises, I regularly run into questions like the following: if the bass has the CF, the alto is in 2nd species and the soprano in 3rd species, how exactly should I evaluate the counterpoint between the alto and the soprano? Is this just a version of two-part 2nd species in halved note values, or should I allow myself more leeway? If the latter, how much leeway?
Of course there are going to be obvious exceptions. Such as that the interval of the perfect fourth is allowable between the upper voices in a chord of the sixth. But other than such modifications, which rules should I abide by when evaluating the counterpoint between the upper voices?