First of all, Tolen Mar, I admire your attitude. This place is a fantastic resource for GMing ideas and my skills as a GM have improved more in the last 2 years of being on the ENWorld boards than any time in the last 20 years that I've been doing it.
I've never been in the situation of running a 3E campaign with a large party, but a few things seem obvious to me. At low level, putting in a few extra goblins, orcs or kobolds (or other creatures who are primarily a threat because of their melee or missile combat capabilities) will balance things out fine. After that, you are just going to have to look hard at the creatures that you use as opposition because it depends a great deal on the capabilities of the party.
For example, a trio of Ogres might cause serious damage to a group of three 2nd level Fighters and three 2nd level Rogues. But replace one of the Fighters with a Sorcerer with Sleep and a couple of failed Will saves later, the Rogues are coup de gracing the sleeping Ogres.
Regarding your specific planned encounters, I would say the same thing. Does the party have a means of taking out several of the goblins at one time from range? Low level arcane magic is good for this and if your party arcane caster (assuming there is one) could take out most or all of the goblins guarding the bridge with a Sleep or Color Spray. Toss in a few bow shots from the Fighters and Rogues in the group and no goblins may survive to retreat to the cave.
On the other hand, lacking that capability and if the party isn't well equipped with ranged weapons, the goblins will be in a good position to defend themselves with their bows. If they roll well and the party isn't in "defensive mode" yet (shields readied or AC enhancing spells in place), they could easily take down one or two party members on the first round. Things could get real ugly after that.
My personal philosophy for starting a campaign is to begin with a few very small encounters that are heavily in the favor of the party. It lets the group try out the characters when a few tactical errors are unlikely to cost them their lives and it gives them a chance to figure out their role in the party dynamic. Then I move to a more serious encounter to put the group to the test but make sure they have the possibility of retreat if things go badly. Sprinkle in a few traps or roleplaying encounters and by then they have probably reached 2nd level.
Once the group is 2nd level, they should have enough cushion in terms of hit points and healing capabilities that one small bad decision or one round of bad rolls is probably not going to spell total disaster (although several bad decisions and lots of bad luck will probably result in some deaths).
So I guess what I'm saying is that part of being a good GM is paying close attention to the abilities of the party and designing encounters that will sometimes be easy, usually test them and occasionally push them to (or perhaps beyond) their limits.