Timid isn't worth it on Mega Venusaur. It needs the bulk more than it needs the Speed. 252 HP/236 Def/20 SDef is the preferred spread. However, Venusaur isn't the best Mega to use in doubles formats, as it generally performs better in singles and there are better Mega Pokemon to use in doubles. Consider Amoonguss over Mega Venusaur because it has better support capabilities:
-Giga Drain
-Spore
-Rage Powder
-Protect
Nature: Calm/Sassy
EVs: 252 HP/180 Def/76 SDef
Item: Black Sludge/Rocky Helmet
Roost isn't ideal in doubles because of the fast-paced format. There's a risk that Talonflame will be double-targeted the turn it Roosts up, and given its frailty, will be KOed if that happens. Flare Blitz is also a mandatory option on Talonflame so it can muscle past Steel Pokemon:
-Flare Blitz
-Brave Bird
-Tailwind/Quick Guard
-Protect/Quick Guard
Nature: Adamant
EVs: 84 HP/252 Atk/172 Spe
Item: Life Orb/Sharp Beak/Sky Plate
Ability: Gale Wings
Mega Manectric is preferred over regular Manectric because of the Intimidate support it brings. Though Raichu is frail as well, it brings better utility options to the table and packs better Speed than vanilla Manectric:
-Thunderbolt/Volt Switch
-Fake Out
-Encore
-Protect/Hidden Power (Ice)
Nature: Timid
EVs: 36 HP/252 SAtk/220 Spe
Item: Focus Sash
Ability: Lightning Rod
Stealth Rock also isn't ideal on doubles formats, due to the infrequency of switching around. Special Tyranitar seems weird at first glance, but it's preferred over physical Tyranitar in the same vein as Salamence. Even with a -Attack nature, Stone Edge and Rock Slide still hurt Tyranitar's targets hard. Physical Tyranitar should only really be used as a Dragon Dance sweeper. Even though Dragon Dance, a setup move, isn't ideal in doubles, you have Amoonguss as Rage Powder support, which I outlined earlier.
Read this article for more info:
-Rock Slide/Stone Edge
-Dark Pulse/Ice Beam
-Fire Blast/Ice Beam
-Protect
Nature: Modest
EVs: 4 HP/252 SAtk/252 Spe
Item: Life Orb
Ability: Sand Stream
or
-Dragon Dance
-Rock Slide
-Crunch
-Protect
Nature: Jolly/Adamant
EVs: 252 Atk/4 Def/252 Spe
Item: Tyranitarite/Weakness Policy/Lum Berry
Ability: Sand Stream
or
-Rock Slide/Stone Edge
-Dark Pulse
-Fire Blast
-Ice Beam
Nature: Timid
EVs: 252 Atk/4 Def/252 Spe
Item: Choice Scarf
Ability: Sand Stream
Since you now lack a way to deal with Fairies, really, Scizor is also a viable candidate over Tyranitar:
-Swords Dance
-X-Scissor
-Bullet Punch
-Protect
Nature: Adamant
EVs: 252 HP/252 Atk/4 SDef
Item: Scizorite/Lum Berry
Ability: Technician