The -or ending for some first and second declension adjectives like maior and minor can be used with both masculine and feminine words. Can that ending also be used with neuter words or would another ending or word be used instead?
Ursus Minor / Ursus Maior
Ursa Minor / Ursa Maior
In other words, does that same ending also work for this:
Pirum Maior / Pirum Maior
There are three types of third-declension (genitive in -is) adjectives in Latin, with nicely descriptive names.
Three-termination adjectives look different in all three genders, like acer (m), acris (f), acre (n). These ones almost exclusively have an -r in the masculine, and they'll have three forms listed in the dictionary (acer, acris, acre). There are very few of these.
Two-termination adjectives look the same in the masculine and feminine, but different in the neuter, like omnis (m/f), omne (n). The vast majority of third-declension adjectives fall into this category, and most of them will have -is in the masculine and feminine and -e in the neuter (omnis, gravis, similis…). Comparatives fall into this category; they have -or in the masculine and feminine and -us in the neuter. So while you have ursa major and canis major, it's pirum majus. These ones will have two forms listed in the dictionary (omnis, omne).
One-termination adjectives, finally, look the same in all three genders. These ones usually end in -x, like felix (m/f/n): vir felix, mulier felix, ferrum felix. These ones are also fairly rare, but all present participles fall into this category: vir amans, mulier amans, ferrum amans. In the dictionary, these ones will list the nominative and genitive, just like a noun: felix, felicis.
Maior and minor are what's called "two-termination" 3rd declension adjectives. This means that masculine and feminine take one form and neuter another. You'll be able to tell this by looking at the dictionary. For these two words, they're both comparatives, so you'll have to look under the positive entry. In this case, it's magnus and parvus respectively. Therein you'll see maior, -us and minor, -us listed. The first form is for masculine/feminine nouns and the second is for neuter nouns.
So, if you have pirum in the nominative or accusative singular, the corresponding comparative form for magnus/parvus will be maius/minus.
For more on third declension adjectives, see Allen and Greenough's section on the topic.
Minor and maior are comparatives of parvus and magnus, respectively.
All adjectives look and behave the same in comparative and superlative, irrespective of the underlying declension. Comparatives are always in the third declension of the two-termination kind, and superlatives are always of the first and second declension declining just like bonus.
In the comparative the singular nominatives end in -ŏr (m&f) and -ŭs (n) and genitives in -ōris (m&f&n). This holds for minor and maior just like altior or junior.
Thus your pirum would be maius or minus.
I'll keep it very simple: no, the neuter (nom./acc.) of comparatives always ends on -(i)us.
pirum minus "the smaller pear (of two), a pear smaller (than some other)"
pirum maius
pirum vetustius
Note that the neuter nominative of a comparative is also used as the comparative adverb.
vivebat longius "she lived longer"
