Play Suikoden IV/Tactics to know of the relation between the elements
That is absolutely NOT what is going on. First of all, if that was the case, these games would suck because you'd have to know rules that hadn't even been invented in order to understand what was going on. The reasoning has to make sense in its own context.
Secondly, Wind is not greater or more powerful than Earth. That's just game mechanics for S4. It has had no bearing whatsoever on any of the rest of the series, whether before or after (i.e. S5).
Third, and actually to the point of the question: You have to have some power over someone in order to steal their True Rune. Geddoe, Sasarai, and Luc are all equal in power. They've all had their runes for a long time. Sasarai and Luc were created to bear their runes. Hugo had the spirit of the Flame Champion (Luc already tried to steal the rune in the Flame Champion's Hideout and wasn't able). Chris doesn't have a long or mystic connection to her rune, so she is easily the least powerful of the five. Since Luc is equal to Geddoe, Sasarai, and Hugo, he needs some edge in the power struggle. For Hugo and Geddoe, that means getting a second True Rune (it doesn't matter which one it is).
Luc first tries to get the runes from their sealed hiding places. That is far easier than stealing it from a person. His plan was to get Fire and Water first, then steal Lightning, then steal Earth. Throughout the game, he is thwarted from getting the first two. Thus, he has to steal a True Rune from the one person that he knows he has power over. To him, this is obviously Sasarai, because he knows his and Sasarai's origins, while Sasarai does not. He knows that Sasarai will be defenseless against his attempt once he finds out the truth.
After he gets Earth, he can get the others. He lures Geddoe away from the rest and takes his first (using Earth because he knows that Geddoe has a lot of control and power over the rune). Then he lures Chris away and has Sarah (who has much greater magical strength than Chris, and therefore does not need another rune) steal hers. At the same time, he steals Hugo's using another rune because he's already tried and failed using his Wind rune alone.
So once again, as Raww, KFCrispy and I have already told you, there aren't any special rules about how many runes you need or which rune is stronger than another. It's a simple power struggle.