If you're playing Arbor Elf/Garruk Wildspeaker and using them to generate tons of mana via enchanted lands, Voyaging Satyr, Fertile Ground and Overgrowth are going to be stronger than Farseek and Dragonlord's Servant.

I also don't think you want Taurean Mauler. It's not going to get that big and more likely your opponent will nuke it with removal. A 4th. copy of Scaleguard Sentinels is going to be stronger.

You're also missing Lightning Bolt from the maindeck. It's one of the reasons to play red in Modern. You only want it in the sideboard if you're playing a combo deck, like Storm. Otherwise, for every control, midrange, or aggro deck, it goes in the main.

Regarding the sideboard, Flame Slash is just worse than Roast since Roast was printed because of Tasigur, the Golden Fang, Siege Rhino, and Tarmogoyf. Ratchet Bomb is going to be slower than playing other types of removal. Against tokens, Pyroclasm, Anger of the Gods, Engineered Explosives, and Firespout are going to be more efficient. These should free up some sideboard slots (after you move Lightning Bolt to the main), giving you room for Ancient Grudge, Molten Rain, etc.

Other than that, the list looks solid.

Thanks for the feedback.

I included Taurean Mauler in the deck to have more dragons for dragon-dependent cards like Scaleguard Sentinels and Draconic Roar.
And also to fill the 3-drop slot to smooth out the mana-curve in the deck.It also has synergy with Dragonlord's Servant and can be "protected" by Thunderbreak Regent's ability.

The only alternative I see for it is Chameleon Colossus which is good but costs more mana, should go with it instead ?

The deck is all about the dragons as the top-end so Dragonlord's Servant seems to fill the ramp/cost-reduction role nicely while providing an ok-body as well which is why it seems better than more enchantments here and a better blocker (can block a 2/1) than Voyaging Satyr/Sakura-Tribe Elder.
It can also prove useful if I manage to "Ultimate" Garruk Wildspeaker.

If I to include L.Bolt in the main what should I take out ? Maybe Farseek ?

Should I include Harmonize in the maindeck ?
Sometimes I have more mana than things to cast and it can help with that.

About the sideboard:

1. Flame Slash can kill Restoration Angel, Thought-Knot Seer, Loxodon Smiter, Wilt-Leaf Liege and 3/4 Goyf which is not bad and costing less mana seems reasonable.

2. I like the flexibility (killing different cmc permanents) and the option of instant-speed activation of Ratchet Bomb.
   Engineered Explosives is good too but rather limited in a 2-color deck.
   Would Clan Defiance be a good sb option ? I like the potential to hit 2 creatures+the opponent with it ?