I have a Monk and a Beastlord. The pet tanks fine in most situations. What do you recommend for a 3rd? Tank? Bard? Chanter?
SK or warrior. SK is better overall, warrior is easier. Warriors also scale really well with beastlords because most of a warrior's DPS is auto-attack, which is helped dramatically with dichotomic fury.
If you're going with a tank, I'd go with Warrior as Tucoh mentions; my BL pet seems to tank decently, so I don't feel the need to play a tank as well. Otherwise, I'd go Bard. Even with /melody and autoattack, you can get a lot out of them. A few macros to cast their other abilities, and you're set. Since Monk/BL is already fairly busy (I run the same combo myself), I would look to keep it simple, and a bard gives you a lot for a little work.
I can see why people want you to add a tank but the weakness in your team is actually the lack of a player healer. You can heal the pet with the beast, but no one's going to be able to keep your monk or beast up, except mercenaries, and mercenaries are relatively bad. In high difficulty situations, usually involving lots of spell damage to multiple members of the team, a tank is not going to help you. A healer is. But which healer? Given the physical focus of your team, a shaman is an obvious choice. You could also consider a cleric, since you already have a beast. Either way, this is the best way to improve the general power of your team, but whether you go for it depends on whether you actually enjoy playing healers.
Not sure he can do progressiom without SK/war/ench. Mercs are fine, block slow heals and max out their heal AA and heal gear
