don't know if trolling or not, but just in case of the later, there is an icon that looks like a list to the right of all the boxes in the LFG menu that you can click and select individual heroics, just be warned that if you do not do it as a complete random, (having all selected) the daily rewards listed in the LFG menu will be unavailable to you. Hope this is not a troll post as I just woke up from a long night and it's kinda hard to tell serious posts from trolls with a hang over
You're talking about Flashpoints. The OP is talking about Heroics. Both are available in the GF tool.
And yes, it would be nice to use the Group Finder to get a group for a specific heroic. However, I would imagine your queue times would be very very long as it would depend on 3 other people queuing for that exact same heroic.