Q1: Can I use this card's Main effect to place this Tamer and 1 Gargomon and 1 Rapidmon from my trash as my Terriermon's bottom digivolution cards, but then choose to not digivolve that Digimon into MegaGargomon in the hand?
Yes, you can.
Q2: If I have multiples of Terriermon, can I use this card's Main effect to place this Tamer and 1 Gargomon and 1 Rapidmon from my trash in the digivolution cards of different Terriermon?
No, you can't. This effect only allows you to place this card and 1 Gargomon and 1 Rapidmon from your trash in the digivolution cards of 1 Terriermon.
Henry Wong ST17-10 R [Start of Your Main Phase] If your opponent has a Digimon, gain 1 memory. [Main] By placing this Tamer and 1 [Gargomon] and 1 [Rapidmon] from your trash in any order as one of your [Terriermon]'s bottom digivolution cards, that Digimon may digivolve into [MegaGargomon] in the hand for a digivolution cost of 4, ignoring its digivolution requirements. If this effect digivolved, that Digimon gains <Rush> for the turn. --- [Security] Play this card without paying the cost.
Henry Wong ST17-10 R [Start of Your Main Phase] If your opponent has a Digimon, gain 1 memory. [Main] By placing this Tamer and 1 [Gargomon] and 1 [Rapidmon] from your trash in any order as one of your [Terriermon]'s bottom digivolution cards, that Digimon may digivolve into [MegaGargomon] in the hand for a digivolution cost of 4, ignoring its digivolution requirements. If this effect digivolved, that Digimon gains <Rush> for the turn. --- [Security] Play this card without paying the cost.
Henry Wong ST17-10 R [Start of Your Main Phase] If your opponent has a Digimon, gain 1 memory. [Main] By placing this Tamer and 1 [Gargomon] and 1 [Rapidmon] from your trash in any order as one of your [Terriermon]'s bottom digivolution cards, that Digimon may digivolve into [MegaGargomon] in the hand for a digivolution cost of 4, ignoring its digivolution requirements. If this effect digivolved, that Digimon gains <Rush> for the turn. ---
李健良 ST17-10 R 【自分のメインフェイズ開始時】相手のデジモンがいるなら、メモリー+1。 【メイン】このテイマーと自分のトラッシュの「ガルゴモン」と「ラピッドモン」1枚ずつを好きな順番で自分の「テリアモン」の進化元の下に置くことで、そのデジモンを手札の「セントガルゴモン」に進化条件を無視して進化コスト4で進化できる。この効果で進化したなら、ターン終了まで、そのデジモンは≪速攻≫を得る。 --- 【セキュリティ】このカードをコストを支払わずに登場させる。