Rulings
Q1: Does this card's "cards can't be added to security stacks by your opponent's effects" mean that effects that add cards to security stacks on "your opponent's Digimon with play costs of 7 or less" can't be activated?
No. Cards can't be added to security stacks by your opponent's effects regardless of the card type or level (the effects will activate, but cards still can't be added to security stacks). The "your opponent's Digimon with play costs of 7 or less" refers to the "can't attack players" effect.
Q2: What happens if my opponent activates this card's effect and I use ST10-14 Chaos Degradation?
Due to this card's effect, cards can't be placed in security stacks, therefore ST10-14 Chaos Degradation's "place 1 of your opponent's Digimon face down at the top or bottom of your opponent's security stack" can't be activated, and the target Digimon will be left as-is.
Q3: While my opponent has this card's effect activated, I used BT10-086 Omnimon (X Antibody)'s effect to reveal all of my opponent's security cards. After choosing and trashing 1 card among them, can the rest be placed in the security stack?
Yes, the rest are placed in the security stack. When returning cards that were revealed from a private area such as the deck or security stack by an effect, it isn't considered adding cards to such areas, therefore this card's effect doesn't apply.
Data Sources
Kongou BT9-103 C [Main] Until the end of your opponent's turn, your opponent's Digimon with play costs of 7 or less can't attack players, and cards can't be added to security stacks by your opponent's effects. --- Inherited: -
Kongou BT9-103 C [Main] Until the end of your opponent's turn, your opponent's Digimon with play costs of 7 or less can't attack players, and cards can't be added to security stacks by your opponent's effects. --- Inherited: [Security] Activate this card's [Main] effect.
Kongou BT9-103 C [Main] Until the end of your opponent's turn, your opponent's Digimon with play costs of 7 or less can't attack players, and cards can't be added to security stacks by your opponent's effects.
金剛 BT9-103 C 【メイン】次の相手のターン終了まで、登場コスト7以下の相手のデジモン全てはプレイヤーにアタックできず、相手の効果でお互いのセキュリティは増やせない。 --- Inherited: 【セキュリティ】このカードの【メイン】効果を発揮する。